1. Field of the Invention
The present invention relates generally to a purity adjustment device for a video display appliance such as a television receiver or a color monitor. In particular, the present invention relates to a purity adjustment device for a video display appliance which can automatically compensate for the purity of the picture that is varied due to the influence of the earth magnetism by electromagnetic deflection using a purity coil.
2. Description of the Related Art
Generally, in order to clearly display RGB (red, green, blue) color signals on a CRT (cathode ray tube) screen of a video display appliance, a purity adjustment is essentially required. Such a purity adjustment is performed in such a manner that the RGB color signals are scanned on one point of the screen, i.e., on a pixel of the screen and then the purity is adjusted until the color of the point becomes white.
A conventional purity adjustment device for a video display appliance, as shown in FIG. 1, includes a CRT 71 having electron guns 70 for emitting electron beams for the RGB color signals and displaying a color image on its screen, a centering permanent magnet 72, mounted on the neck portion of the CRT 71, for positioning the electron beam spot on the center of the fluorescent screen of the CRT 71 by its electromagnetic force, and a deflection coil 73, mounted on the neck portion of the CRT 71, for deflecting the electron beams emitted from the electron guns 70 in horizontal and vertical directions.
The operation of the conventional purity adjustment device as constructed above will be explained.
First, when an experimental image signal is inputted to the CRT 71 for the purity adjustment of the CRT 71, electron beams for the RGB color signals corresponding to the input video signal are emitted from the electron guns 70 to the fluorescent screen of the CRT 71. At the same time, the electron beams emitted from the electron guns 70 pass through the region of the centering magnet 72, and thus the spot of the electron beams is positioned on the center of the fluorescent screen by the magnetic field produced from the centering magnet 72. Also, the spot of the electron beams is deflected in the vertical and horizontal directions by the deflection coil 73 to scan the fluorescent screen of the CRT 71, resulting in that the fluorescent screen of the CRT 71 produces a color which corresponds to the electron beams for the RGB color signals being scanned on the fluorescent screen. At this time, an examiner may observe the color displayed on the screen of the CRT 71, and adjust the purity by adjusting the position of the deflection coil 73 and the centering magnet 72 so that the color displayed on the screen presents white.
However, according to the conventional purity adjustment device as described above, since the purity is adjusted so that the electron beam spot is positioned on the center of fluorescent screen of the CRT 71 by the centering magnet 72 mounted on the neck portion of the CRT 71, the purity should be newly adjusted if the direction of the earth magnetic field is changed. Specifically, if the use environment of the video display appliance is changed, for instance, if the video display appliance whose purity adjustment has been made in the Southern Hemisphere is to be used in the Northern Hemisphere, the position of the electron beam spot scanned on the CRT screen will be changed due to the change of the earth magnetic field, whose direction in the Southern Hemisphere is opposite to that in the Northern Hemisphere, thereby deteriorating the purity of the picture displayed on the CRT screen.